Wow. I am so glad I read this (I almost gave it a miss because of the violence warning). It's fantastic.
Aberforth's conflicted relationship with Albus rings very true, as does his response to Snape's pain. In the books Aberforth is an intriguing character because he claims that Harry's on the losing side and there's likely no point in fighting -- yet he himself helps the DA students anyway. That tension comes through strongly in this story. He's a realist, but not a cynic. It's not that he has hope, it's more that he sees it as worthwhile to treat others with compassion even when there is no hope. That's the core of who he is, and it sets him apart from his brother, who (IMO) sees other people not necessarily as unimportant, but certainly as less important than achieving the greater good. Aberforth does right, but doesn't become so damned much of a Hero that he forgets the human side of things.
Your Snape is wonderful. I've often thought that Snape was someone who struggled to grow up, because of his circumstances being dependent on a father figure of sorts and constantly immersed in the childhood setting of Hogwarts, with no opportunity to get away from it all and gain perspective on his early life. I think there's some of that in your Snape here, but you've captured him at the moment when the protection of that "father" is stripped away, and he's left to rely on himself. As it turns out, he can do it -- he always could. But he's responding to it like a bird kicked out of the nest for the first time, still not fully believing in his own ability even as he's demonstrating it.
Although he isn't in the story in the strictest sense, I also think you've also got a vivid and perceptive Albus characterization here, drawn in negative space, as it were. By seeing how these two men saw him, his outline comes through clear as day.
As Donna pointed out, your style is very painterly. Every detail adds to the ominous and oppressive time these characters live in. I'm also delighted by your use of different names for the characters as the scenes go on, pointing out the different roles they play, which all come together to form their entire selves.
I'm really blown away by this and can't wait to find out who's responsible for such a flawless piece!